A converter (200) adapted to convert an analog input signal into a digital output signal includes an analog input terminal (205) for receiving the analog input signal, a redundant Signed digit (rsd) stage (210) coupled to the analog input terminal, and a digital section (220). The rsd stage is configured to receive the analog input signal at the analog input terminal, produce a first number of bits at a digital output from the analog input signal during a first half of a first clock cycle, provide a residual feedback signal of the analog input signal at the analog input terminal during a second half of the first clock cycle, and produce a second number of bits at the digital output from the residual feedback signal during a first half of a second clock cycle, the second number of bits less than the first number of bits.
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14. In a cyclic redundant signed digit (rsd) analog-to-digital converter (ADC) having a multiplying digital-to-analog converter (MDAC), the MDAC using at least one amplifier during an analog-to-digital conversion cycle comprising a plurality of clock cycles, a method for converting an analog input signal to a digital output signal, the method comprising:
receiving the analog input signal;
comparing a residual feedback signal to a reference voltage;
amplifying the residual feedback signal during the first clock cycle of the plurality of clock cycles using the at least one amplifier configured to have a gain factor of at least four;
amplifying the residual feedback signal during all clock cycles of the plurality of clock cycles subsequent to the first clock cycle using the at least one amplifier configured to have a gain factor of two and further configured to reduce a current in the at least one amplifier by a factor of at least four; and
generating a plurality of digital bits that are proportional to the analog input signal.
1. A cyclic redundant signed digit (rsd) analog-to-digital converter (ADC) comprising:
a gain circuit, comprising:
a first switch coupled between an input terminal and a first node, the first switch for applying an input signal to the first node;
a second switch coupled between the first node and a second node, the second switch for applying a residual voltage feedback signal to the first node;
a first amplifier having a first input terminal, a second input terminal, and an output terminal selectively coupled to the second node;
a second amplifier having a first input terminal, a second input terminal, and an output terminal selectively coupled to the second node; and
at least first, second, third, and fourth capacitors, each of the first, second, third, and fourth capacitors capable of being selectively coupled between the first node and the first input terminals of both of the first and second amplifiers during an analog-to-digital conversion comprising a plurality of clock cycles;
wherein the gain circuit uses the first amplifier to generate a gain factor of at least four and the gain circuit uses the second amplifier to generate a gain factor of two, wherein the first amplifier operates to amplify the residual voltage feedback signal during the first clock cycle of the plurality of clock cycles while the second amplifier is disconnected from the first node, and wherein during a second clock cycle subsequent to the first clock cycle, the second amplifier operates to amplify the residual voltage feedback signal while the first amplifier is disconnected from the first node.
9. A cyclic redundant signed digit (rsd) analog-to-digital converter (ADC) for converting an analog input signal to a digital output signal, the rsd ADC comprising:
an analog input terminal for receiving an analog input signal;
an rsd stage coupled to the analog input terminal, the rsd stage comprising:
a plurality of comparators coupled to the analog input terminal and configured to compare a residual feedback signal to a plurality of predetermined voltages;
a logic circuit coupled to the plurality of comparators and configured to generate a first number of bits based at least upon outputs from a first set of the plurality of comparators, the logic circuit further configured to generate a second number of bits based at least upon outputs from a second set of comparators, the second set of comparators being a subset of the first set of comparators;
a first amplifier having a first input terminal, a second input terminal, and an output terminal selectively coupled to the second node;
a second amplifier having a first input terminal, a second input terminal, and an output terminal selectively coupled to the second node; and
at least first, second, third, and fourth capacitors, each of the first, second, third, and fourth capacitors capable of being selectively coupled between the first node and the first input terminals of both of the first and second amplifiers during an analog-to-digital conversion comprising a plurality of clock cycles;
wherein the rsd stage has a gain factor of at least four when using the first amplifier, and the rsd stage has a gain factor of at least two when using the second amplifier, wherein the first amplifier operates to amplify the residual voltage feedback signal during a first clock cycle of the plurality of clock cycles while the second amplifier is disconnected from the first node, wherein during a second clock cycle subsequent to the first clock cycle, the second amplifier operates to amplify the residual voltage feedback signal while the first amplifier is disconnected from the first node, and wherein the second amplifier operates to amplify the residual voltage feedback signal during all remaining clock cycles of the plurality of clock cycles; and
a digital section coupled to the logic circuit, the digital section configured to perform a digital alignment and correction on the first number of bits and the second number of bits to generate the digital output signal.
2. The rsd ADC of
3. The rsd ADC of
a plurality of comparators coupled to the input terminal and configured to compare the residual feedback signal to a plurality of predetermined voltage;
a logic circuit coupled to the plurality of comparators and configured to generate a first number of bits based at least upon outputs from a first set of plurality of comparators, the logic circuit further configured to generate a first number of bits based at least upon outputs from a first set of the plurality of comparators, the logic circuit further configured to generate a second number of bits based at least upon outputs from a second set of comparators, the second set of comparators being a subset of the first set of comparators; and
a digital section coupled to the logic circuit, the digital section configured to perform a digital alignment and correction on the first number of bits and the second number of bits to generate the digital output signal.
4. The rsd ADC of
5. The rsd ADC of
8. The rsd ADC of clam 1, wherein when the first switch is closed, the second switch is open, and wherein when the second switch is closed, the first switch is open.
11. The rsd ADC of
12. The rsd ADC of
13. The rsd ADC of
16. The method of
17. The method of
18. The method of
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This disclosure relates generally to Analog-to-Digital (A/D) converters, and more particularly to Redundant Signed Digit (RSD) A/D converters with current reduction.
Advances in integrated circuit technology have enabled the development of complex “system-on-a-chip” ICs for a variety of applications such as wireless communications and digital cameras. Such applications are embodied in portable electronic devices for which low power and small circuit area are important design factors. Low power and low voltage circuits are needed to decrease battery power requirements, which can allow for designs that use fewer or smaller batteries, which in turn decreases device size, weight, and operating temperature.
Such devices, however, receive analog input signals that are typically converted to digital signals. Various conventional cyclic (algorithmic) A/D converters that achieve relatively lower power operation and with a sufficiently high resolution in a relatively small area have been achieved.
Even lower power while achieving the needed resolution is a continuing desire. Thus any further reductions in power are desirable. Accordingly there is a need for further reductions in power while achieving the needed resolution.
The following detailed description of some example embodiments will be better understood when read in conjunction with the appended drawings. It should be understood, however, that example embodiments are not limited to the precise arrangements and instrumentalities shown. In the drawings, like numerals are used to indicate like elements throughout. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring aspects of the illustrated embodiments. Additionally, elements in the drawings figures are not necessarily drawn to scale. For example, the dimensions of some of the elements or regions in some of the figures may be exaggerated relative to other elements or regions of the same or other figures to help improve understanding of the example embodiments. In the drawings:
The detailed description set forth below in connection with the appended drawings is intended as a description of some of the example embodiments, and is not intended to completely describe all possible embodiments. That is, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, or the following detailed description of example embodiments. It is to be understood that the same or equivalent functions may be accomplished by different embodiments.
The terms “first,” “second,” “third,” “fourth” and the like in the description and the claims, if any, may be used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments described herein are, for example, capable of use in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have” and any variations thereof, are intended to cover non-exclusive inclusions, such that a process, method, article, or apparatus that comprises, includes, or has a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
The architecture of
The RSD stage 300 further includes a first, second, third, fourth, fifth, and sixth comparators 302, 304, 306, 308, 310, and 312, respectively. Because the RSD stage 300 has six comparators, it can achieve a maximum resolution of 2.5 bits. Although the six-comparator configuration illustrated in
Each of the comparators 302, 304, 306, 308, 310, 312 also has a negative input terminal that receives a first, second, third, fourth, fifth, and sixth predetermined voltage signals, respectively (e.g., VREF1, VREF2, VREF3, VREF4, VREF5, and VREF6). Each of the first, second, third, fourth, fifth, and sixth comparators 302, 304, 306, 308, 310, and 312 compare the signals applied to their respective input terminals to generate a comparator output signal.
According to an example embodiment, the RSD stage 300 is configurable such that, during an A/D conversion process for an analog input signal that occurs over a number of sequential clock phases, the values of the predetermined voltage signals (VREF1, VREF2, VREF3, VREF4, VREF5, and VREF6) may be selectively changed for each one of the clock phases. For example, during a first clock phase of the analog to digital conversion, each of the first, second, third, fourth, fifth, and sixth predetermined voltage signals (VREF1, VREF2, VREF3, VREF4, VREF5, and VREF6) may each be set to a unique value. During second and subsequent clock phases of the analog to digital conversion, some or all of the first, second, third, fourth, fifth, and sixth predetermined voltage signals (VREF1, VREF2, VREF3, VREF4, VREF5, and VREF6) may be changed to have a different value then in a previous clock phase.
According to the example embodiment, during clock phases of the A/D conversion after the first clock phase, the RSD stage 300 uses outputs from less than all of the comparators 302, 304, 306, 308, 310, and 312. In other words, for clock phases after the first clock phase, the resolution that is achieved from the single-bit/multi-bit RSD stage 300 is reduced relative to the resolution of the first clock phase. These aspects of the example embodiment are described in greater detail below.
The outputs of the first, second, third, fourth, fifth, and sixth comparators 302, 304, 306, 308, 310, and 312 are connected to a logic circuit 320. During clock phases of an A/D conversion process, the logic circuit 320 is capable of generating a digital output signal that is representative of the selected one of either the analog input signal or the residual voltage feedback signal. In an example embodiment, the logic circuit 320 generates three raw digital bits (D0, D1, D2) as the digital output signal during a clock phase of an A/D conversion process based upon the output from all the comparators 302, 304, 306, 308, 310, 312. In the example embodiment, the logic circuit 320 generates two raw digital bits (D0, D1) as the digital output signal during another clock phase of the A/D conversion process based upon outputs from less than all of the comparators 302, 304, 306, 308, 310, 312. In a preferred embodiment, the three digital bits (D0, D1, D2) are generated during the first clock phase of the A/D conversion process. The digital bits generated during any clock phase of the A/D conversion are aligned and synchronized in the digital section 220, and then combined with the digital bit or bits from other clock phases of the A/D conversion to form a formatted binary output code.
During clock phases of the A/D conversion, the logic circuit 320 is also capable of generating a high switch control signal 333, a mid switch control signal 353, and a low switch control signal 343 based upon at least two of the output signals from the comparators 302, 304, 306, 308, 310, and 312.
The single-bit/multi-bit RSD stage 300 additionally includes a programmable gain/summing element 325. The programmable gain/summing element 325 receives as inputs the high switch control signal 333, the mid switch control signal 353, the low switch control signal 343, the selected one of the analog input signal and the residual voltage feedback signal from node 307, a first reference voltage VREFP, a second reference voltage VREFM, and a ground voltage. The programmable gain/summing element 325 generates the residual voltage feedback signal VR. Although the actual transfer function associated with the programmable gain/summing element 325 will be dependent upon the particular design, generally speaking, the residual voltage feedback signal VR may be thought of as a sum of two products. Depending on the particular clock phase of the A/D conversion, the first product is either the analog input signal or a previously generated value of the residual voltage feedback signal, multiplied by a first gain factor. The second product is a selected one of the reference voltages (VREFP, VREFM, or zero), multiplied by a second gain factor.
The feedback switch 315 is provided for selecting the residual voltage feedback signal as an input to the programmable gain/summing element 325 and the comparators 302, 304, 306, 308, 310, and 312. The feedback switch 315 is disposed between the output of the programmable gain/summing element 325 and the node 307. When the feedback switch 315 is closed, the first switch 305 is open so that the residual voltage feedback signal is input to the programmable gain/summing element 325 and the comparators 302, 304, 306, 308, 310, and 312. When the first switch 305 is closed, the feedback switch 315 is open so that the analog input signal is input to the programmable gain/summing element 325 and the comparators 302, 304, 306, 308, 310, and 312. As discussed above, the first switch 305 is closed in a first clock cycle during the conversion of the analog input signal and the first switch 305 is open for subsequent cycles of converting the analog input signal.
Referring to
The sub-ADC 400 further includes first, second, third, fourth, fifth, and sixth comparators 302, 304, 306, 308, 310, and 312, respectively. The operation of the comparators 302, 304, 306, 308, 310, 312 is the same as what was described above for
During the first clock phase of an A/D conversion process, the logic circuit 320 generates three raw digital bits (D0, D1, D2) based upon the output signals from each of the first through sixth comparators 302, 304, 306, 308, 310, 312. According to the example embodiment, the logic circuit 320 generates two raw digital bits (D0, D1) during one or more subsequent clock phases of the A/D conversion process, based upon the output signals from less than all of the comparators 302, 304, 306, 308, 310, 312. The logic circuit also generates control signals (h, l, m), which are used to control some switches of the MDAC 325. This will be explained in further detail below. The control signals h, l, m correspond to the high, low, and mid switch control signals 333, 343, and 353 of
The programmable gain/summing element 325 of
The circuit connections illustrated in
According to an example embodiment, during a clock phase of an A/D conversion process the MDAC 325 is operable to produce a first gain factor of four (4) for the analog input signal and a selected second gain factor of zero, one, two, or three (0, 1, 2, or 3) for a selected one of the reference voltages (VREFP, VREFM, or zero). According to the example embodiment, during subsequent clock phases of the A/D conversion the MDAC 325 is operable to produce a first gain factor of two (2) for the residual voltage feedback signal and a selected second gain factor of zero or one (0 or 1) for a selected one of the reference voltages (VREFP, VREFM, or zero).
Switches 305 and 315 of
The switches that are included in the MDAC 325 of
TABLE I
Switch(es)
Control Signal
502, 504, 506, 508
SWVIN
512, 514
SWFB1
522, 524
SWFB2
532, 534, 536
h (from logic circuit 320)
542, 544, 546
l (from logic circuit 320)
552, 554, 556
m (from logic circuit 320)
562, 564
MFB1
572, 574
MFB2
582, 584
HO
586
HE
588
HSH
590
SWVIN (OR) SWFB1
592
SWVIN (OR) SWFB2
594
reset
601, 602, 604
SWVIN (OR) HO
603, 605
NOT (SWVIN (OR) HO)
Table II, which appears below this paragraph, illustrates the state of all the switches that are controlled by the control signals of
TABLE II
Clock Phase
Switch(es)
1
2
3
4
5
6
7
8
9
10
502, 504,
X
506, 508
512, 514
X
X
X
522, 524
X
X
X
562, 564
X
X
X
572, 574
X
X
X
582, 584
X
586
X
588
X
X
590
X
X
X
X
592
X
X
X
X
601, 602,
X
X
604
603, 604
X
X
X
X
X
X
X
X
As explained above, the raw digital bits obtained from the sub-ADC 400 in the example 10-bit A/D conversion were sent to the digital section 220 of
According to the example 10-bit A/D conversion described above, five clock cycles are needed to produce a 10-bit binary word. Thus, generalizing to any n-bit A/D conversion where n is even, an n-bit binary word may be produced in n/2 clock cycles. In alternative embodiments, the sub-ADC 400 and the MDAC 325 could be configured to produce two raw digital bits during, for example, the ninth clock phase that was described above as producing three raw digital bits for the example 10-bit A/D conversion. Thus, generalizing to any n-bit A/D conversion where n is odd, an n-bit binary word may be produced in (n+1)/2 clock cycles. The number of clock cycles used in the example 10-bit conversion described above is not significantly different from the number of clock cycles required by the single multi-bit A/D converter described in U.S. Pat. No. 6,535,157, which may produce two digital bits during one clock phase of every clock cycle. However, those skilled in the art will appreciate that because the sub-ADC 400 and the MDAC 325 of the example embodiment can be continually reconfigured to produce two raw digital bits during every clock phase for a clock cycle after an initial clock cycle, the example embodiment can achieve the same performance with reduced thermal noise, area, and power.
For example, in the 10-bit conversion described above, the first clock phase of the first clock cycle and the ninth clock phase of the fifth clock cycle were used to produce three raw digital bits from the sub-ADC 400. The sub-ADC 400 was not used during the second clock phase of the first clock cycle. In the second through fourth clock cycles, however, by efficiently reconfiguring circuitry in the MDAC 325 during each clock phase to perform a different function, the sub-ADC 400 was used during each clock phase to produce two raw digital bits in each clock phase. Thus, according to example embodiments a single RSD A/D conversion stage can be initially configured to output at least three raw bits during an initial conversion clock cycle, then be subsequently reconfigured to output two raw bits during every clock phase of subsequent conversion clock cycles in order to determine the remaining bits of the A/D conversion with reduced capacitance, reduced area, and reduced power requirements.
Based on the above, it should be apparent that example embodiments include a single RSD stage that can be selectively reconfigured to have different bit resolutions during different clock phases or clock cycles of an A/D conversion process. In the particular example described above, the initial resolution was 2.5 bits, and the subsequent resolution was 1.5 bits.
The example embodiment described above can achieve the same sample rate and resolution as the architecture described in U.S. Pat. No. 6,535,157, but the reconfiguration from a 2.5 bit resolution stage in the first conversion clock cycle to a 1.5 bit resolution stage in subsequent clock cycles as described above enables it to do so with approximately a 40% reduction in total capacitance due to reduced thermal noise and approximately a 25% reduction in area and power.
While the order of processes 1610 and 1620 as illustrated in
Thus, it is seen that a power savings is achieved by reducing current of an amplifier in MDAC at a time when resolution is reduced. For a given conversion of a sample to a digital signal, the amplifier has an initial resolution at a relatively higher current requirement and then switches to a lower resolution at a lower current requirement at a time when there is a drop in the resolution requirement. This is a one time change during a given conversion of a sample to a digital signal.
It should be apparent by know that a converter adapted to convert an analog input signal into a digital output signal has been disclosed. The converter includes an analog input terminal for receiving the analog input signal. The converter includes a Redundant Signed Digit (RSD) stage coupled to the analog input terminal. The RSD stage is configured to receive the analog input signal at the analog input terminal, produce a first number of bits at a digital output from the analog input signal during a first half of a first clock cycle, provide a residual feedback signal of the analog input signal at the analog input terminal during a second half of the first clock cycle and produce a second number of bits at the digital output from the residual feedback signal during a first half of a second clock cycle, the second number of bits less than the first number of bits. The converter includes a digital section coupled to the digital output, the digital section configured to perform a digital alignment and correction on the first number of bits and the second number of bits to generate the digital output signal. The converter may further comprise a Multiplying Digital to Analog Converter (MDAC) that is operable to produce the residual feedback signal and a subsidiary Analog to Digital Converter (sub-ADC) that is operable to produce the first number of bits based upon the analog input signal and that is operable to produce the second number of bits based upon the residual feedback signal. The converter may further comprise a plurality of comparators coupled to the analog input terminal and configured to compare the residual feedback signal to a plurality of predetermined voltages and a logic circuit coupled to the plurality of comparators and configured to generate the first number of bits based at least upon outputs from a first set of the comparators, the logic circuit further configured to generate the second number of bits based at least upon outputs from a second set of comparators, the second set of comparators a subset of the first set of comparators. The converter may further comprise an operational amplifier (op-amp) operable to generate the residual feedback signal, a first capacitor that is coupled to a first node, a second capacitor that is coupled to the first node, a third capacitor that is coupled to a second node, a fourth capacitor that is coupled to the second node, a first switch that is coupled between an input of the op-amp and the first node, and a second switch that is coupled between the input of the op-amp and the second node. The converter may further comprise a third switch coupled between the first node and the second node. The converter may have a further characterization by which the RSD stage is further configured to produce a third number of bits during a second half of the second clock cycle, the third number equal to the second number. The converter may have a further characterization by which the first number is three and the second number is two.
Also disclosed is a cyclic Redundant Signed Digit (RSD) Analog to Digital (A/D) converter. The converter includes an input terminal for receiving an analog input signal. The converter includes a first switch connected between the input terminal and a first node, the first switch operable to apply the analog input signal to the first node. The converter includes a second switch connected between the first node and a second node, the second switch operable to apply a residual voltage feedback signal to the first node, the first switch operable to be closed when the second switch is open, the second switch operable to be closed when the first switch is open. The converter includes an operational amplifier having an output terminal connected to the second node, the operational amplifier operable to generate the residual voltage feedback signal and apply it to the second node. The converter includes. The converter includes comparators, each comparator having a first input coupled to the first node and an output, each of the comparators operable to compare a selected one of the analog input signal and the residual voltage feedback signal to a predetermined voltage signal. The converter includes a logic circuit coupled to the outputs of the comparators, the logic circuit operable to generate a first digital output signal during a first clock phase of an A/D conversion and operable to generate a second digital output signal during a second clock phase of the A/D conversion, the first digital output signal based upon the outputs from a first set of the comparators, the second digital output signal based upon the outputs from a second set of the comparators. The converter may have a further characterization by which the first digital output signal comprises three digital bits, the second digital output signal consisting of two digital bits. The converter may have a further characterization by which the second clock phase is subsequent to the first clock phase, and wherein there is at least one intervening clock phase between the first clock phase and the second clock phase. The converter may have a further characterization by which the second set of the comparators is a subset of the first set of the comparators. The converter may have a further characterization by which first digital output signal comprises three digital bits and the second digital output signal consists of two digital bits. The converter may have a further characterization by which a clock cycle of the A/D conversion consists of the first clock phase and the second clock phase.
Also disclosed is a method for converting an analog input signal into a plurality of digital bits during a plurality of clock cycles using a single Redundant Signed Digit (RSD) stage of an Analog to Digital (A/D) converter. The method includes receiving the analog input signal. The method includes producing a first number of the plurality of digital bits at a first resolution during one of the clock cycles and producing a second number of the plurality of digital bits at a second resolution during another one of the clock cycles. The method may have a further characterization by which producing the first number of digital bits and the second number of digital bits comprises the steps of producing the first number of the digital bits from the analog input signal during a first half of a first clock cycle, producing a first residual voltage from the analog input signal during a second half of the first clock cycle, and producing the second number of the digital bits from the first residual voltage during a first half of a second clock cycle. The method may have a further characterization by which the first resolution is at least 2.5 bits, and wherein the second resolution is less than the first resolution. The method may have a further characterization by which the second resolution is 1.5 bits. The method may have a further characterization by which the first half of the second clock cycle occurs following the second half of the first clock cycle. The method further comprising the step of producing a second residual voltage from the first residual voltage during a second half of the second clock cycle. The method further comprising the step of producing a third number of digital bits at the second resolution from the second residual voltage during the second half of the second clock cycle.
It shall be apparent to those of ordinary skill, based upon the limited number of example embodiments described above, that many other embodiments that incorporate one or more of the inventive principles that were associated with the described example embodiments exist. In the following paragraphs, more descriptions of example, non-limiting embodiments are presented.
While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist, especially with respect to choices of device types and materials and the sequence of processes. It should further be emphasized that the example embodiments described above are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the detailed description of the example embodiments provides those skilled in the art with a convenient road map for implementing the inventive principles contained in the example embodiments. The inventors regard the subject matter to include all combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. It should also be understood that various changes can be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.
Kabir, Mohammad Nizam U., Braswell, Brandt, Garrity, Douglas A.
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